Exhaust gases generated by combustion in a vehicular engine are directed from the engine through a series of pipes, one or more mufflers, and certain emission control equipment prior to being released into the atmosphere at a safe location on the vehicle. In traveling from the engine to the point where the exhaust is released into the air, the pipes must be circuitously directed around or through other essential components of the vehicle, such as the engine itself, the drive train, the passenger compartment, tanks for fuel or coolant, axles and various structural supports.
The exhaust gases generally are at an elevated temperature and cause the pipes through which they pass to be heated. Frequently it is necessary to physically separate and insulate these heated pipes from other parts of the vehicle or from ambient surroundings. In other instances it may be desirable to pass the exhaust in a heat exchange relationship with cooler air in order to either lower the temperature of the exhaust or to provide heated air for other uses in the vehicle.
In the past the exhaust pipes occasionally have been separated from other parts of the vehicle by heat shields. Heat shields typically have been linear members which are bolted into position intermediate the exhaust pipe and the part of the vehicle to be separated from the heated exhaust. In most instances a gap exists between the exhaust pipe and the heat shield and a second gap exist between the heat shield and the remainder of the vehicle. Two or more opposed heat shields occasionally are used when the exhaust pipe is directed in between two portions of the vehicle which must be separated from the heat.
In certain vehicles it has been found necessary to wind the exhaust pipe circuitously between several vehicular components all of which must be protected from the heat. Space limitations often preclude heat shields in these situations. As a result, in these instances, it has been necessary to employ two generally concentric pipes which extend along the circuitous path through the vehicle. More particularly the inner of the two concentric pipes carries the exhaust from the engine, while the outer pipe separates the heated inner exhaust pipe from the adjacent areas of the vehicle. This structural configuration also enables the air gap between the pipes to perform an insulating function.
Air gap exhaust pipes have been difficult and costly to manufacture. Typically a straight inner pipe with support legs welded to its outer surface is mounted within a straight outer pipe. The support legs maintain the inner and outer pipes in concentric relationship. In certain instances the two straight pipes are concentrically arranged with respect to one another, and dents are formed in the outer pipe to support the inner pipe. To enable concentric bending of the two pipes, a filler then is inserted into the air gap. The filler may either be a granular material, such as sand, or an alloy with a low melting point. With the filler in place, the two pipes then are bent into the desired, circuitous configuration, while still maintaining their concentricity. After the pipes have been bent, the filler is either flushed or melted out.
The above described air gap pipe is expensive and slow to manufacture primarily because of the costs and time required to properly insert and remove the filler. Additionally, to the extent that support legs are used, they tend to perform poorly under conditions of differential thermal expansion and contraction. Specifically if support legs are welded to the inner pipe to provide a secure fit when the pipes are cool, the legs may damage the inner or outer pipe when heat is applied. If the support is provided by dents in the outer pipe, the force exerted to create the dents often will dent both pipes, to either damage the inner pipe or result in a non-concentric alignment.
Attempts have been made to bend the inner and outer pipes separately, and then to utilize a band saw to cut the outer pipe in half along its length. The two halves then were separated and legs were welded to the inner surfaces of the outer pipe halves. The outer pipe halves then were placed around the inner pipe and were welded along the two cut lines. This band saw cutting operation is extremely slow and only can be carried out manually on a piece by piece basis for pipes with simple bends. Consequently this process has been carried out only on very small orders where costs would normally be high in any event. The band saw cuts also tend to be quite rough and must be finished to remove burrs and discontinuous edges. The manual band saw cutting also creates inventory control problems since no two pipes are cut exactly the same. The air gap pipe also has suffered from the above described structural problems caused by expansion and contraction of the legs welded to the inner surface of the outer pipe.
In the past, high energy cutters such as plasma arc and laser cutters have been widely used to cut a variety of shapes into metal pieces. However, neither plasma arc nor laser cutters have been adapted to cut pipes along their longitudinal axis, particularly after the pipes have been bent into complex shapes.
In view of the above, it is an object of the subject invention to provide a method for producing an air gap pipe efficiently and inexpensively.
It is another object of the subject invention to provide a method for producing an air gap pipe in which the inner pipe is efficiently supported within the outer pipe under a broad range of operating temperatures.
It is a further object of the subject invention to provide a method for producing an air gap pipe which will not damage or deform the inner pipe.
It is an additional object of the subject invention to provide an air gap pipe with an improved ability to perform under a broad range of operating conditions.
It is still another object of the subject invention to provide an air gap pipe with an enhanced ability to dissipate heat.